Play it again, brain, but in reverse

Rat brain cells signal in reverse immediately after the animal completes a task, researchers have discovered. They suggest that this type of "reverse replay" in the mind can promote learning and memory.

David Foster and Matthew Wilson at the Massachusetts Institute of Technology in the US exposed four rats to a familiar running track, before exposing them to a new one. The rodents followed each track to eat the food placed at either end.

While the rats ran and then paused after reaching the food, the neurobiologists directly measured the animals' brain cell activity using carefully placed electrodes. They focused on the hippocampus, a region of the brain believed to handle learning and memory.

The recordings revealed that, as the rats ran along each track, the cells in their hippocampus would fire in a particular sequence. But when the animals stopped for food at end of the new track, the same cells would also fire in the opposite order. This reverse-replay did not occur as often when they rested on either end of the familiar track.

The researchers suggest that the reverse signalling etches information into the brains of the rats as part of the learning process.

Other experts say that the new findings could offer an explanation for how the brain associates rewards with actions. The new findings are "absolutely fascinating", says Bruno Pucet at the Laboratory of Neurobiology and Cognition in Marseilles, France.

Hyperactive children

Foster and Wilson say previous studies have demonstrated how rats' hippocampal cells rerun patterns of neural activity during sleep. Scientists have argued that this may help animals consolidate memories.

The discovery of reverse processing in alert rats hints that resting while awake can have benefits for learning and memory too, the authors suggest. They add that taking a break can give the brain a chance to catch up by conducting this type of rewind for new information. "Quiet, wakeful periods may turn out to be important for learning from experience," says Foster.

He adds that it might also give insights into why hyperactive children can sometimes have learning problems: "Learning impairments may have partly to do with something like this - if you don't get enough down time."

Journal reference: Nature (DOI: 10.1038/ nature 04587)

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